Differential current measurement is a technique used in a wide variety of power system applications. For example, the technique is often used in the protection of power system equipment, such as transformers, generators, motors, and the like. Generally, differential current measurement techniques involve monitoring the current at both an input terminal and an output terminal of a device, normalizing the measured input and output currents to compensate for changes in phase and magnitude of the measured currents that may be introduced by the device during normal operation, and then comparing the normalized input and output currents. If the difference between the normalized input and output currents is zero, then the device presumably is working properly. On the contrary, a detected difference between the normalized input and output currents may indicate a fault within the device. In response to the detection of a fault, a differential relay issues a trip command to relevant circuit breakers in order to shut off the power to the protected object and to prevent further damage.
Due to the restructuring of the power industry with the main goal of the liberalization of the electricity market, more strict requirements for control of active and reactive power flow are posed on the electrical utilities. This process indicates that there is an increasing need for appropriate tools to control the power flow within a given network. A most commonly used device for this purpose is a power transformer of special construction often called phase shifting transformer (PST) or phase angle regulating transformer (PAR).
In principle, the technical aspects of a phase shifting transformer are rather simple: It provides a well defined, but changeable phase shift between the primary and secondary terminals. Normally, the phase shift may be varied during operation in definite steps by use of one or more on load tap changers (OLTC). In most cases the sign of the phase shift can be reversed from advance to retard.
Different types of PST designs are presently available. This makes the protection scheme for every type of PST rather unique and thus difficult. Currently, there is no relay manufacturer, which can provide a unique differential protection relay for all of these devices, regardless their design.
Within this area of technology several inventions try to deal with these kinds of problems and some patents have been granted.